The present invention is directed to non-evaporable getter alloys capable of sorbing different gases but particularly useful for the sorption of hydrogen.
Many applications in the field of industry or research require, for their correct working, vacuum or a filling with an atmosphere of a given gas (or gas mixture) in a closed container. Examples are evacuated jackets for thermal insulation (e.g., in thermal bottles, also known as “thermos”, or solar collectors), in which in particular the presence of hydrogen is detrimental owing to the high thermal conductivity of this gas; plasma displays; or X-ray generating tubes. The processes for manufacturing these devices comprise a step of container evacuation and possible back-filling thereof with a desired gas. However, these processes always leave traces of undesired gases in the final device. Besides, in the case of hydrogen, whenever a high vacuum or a hydrogen-free gas are produced, mechanisms exist which cause hydrogen gas to re-enter the system. These mechanisms are mainly the outgassing of the container walls and the hydrogen permeation across these walls from the external atmosphere toward the container, thus leading to problems in the correct operation of the devices. Owing to the same mechanisms, hydrogen also represents the main contribution to the residual pressure in ultra-high vacuum (UHV) systems, such as the particle accelerators employed in the research field.
To remove traces of unwanted gases from evacuated or gas-filled spaces, it is known to employ non-evaporable getter materials (known in the field as NEG materials, or simply as NEGs), i.e., materials being capable of chemically binding molecules of gases such as water, oxygen, hydrogen, carbon oxides and, in some cases, nitrogen. In order to accomplish their function, NEGs generally require an initial treatment of thermal activation at temperatures that can vary between about 300° C. up to about 900° C. during a time comprised between a few minutes up to several hours, depending on the material composition.
NEGs are generally metals of the transition groups III, IV and V or alloys thereof with other elements, generally other transition metals or aluminum. The most used getter materials are titanium- and, particularly, zirconium-based alloys.
NEG materials show a sorption behavior with respect to hydrogen which is different from that of other gases. While for most gases the chemical sorption by these alloys is irreversible, the sorption of hydrogen by NEGs is an equilibrium process reversible as a function of the temperature. Hydrogen is efficiently sorbed at relatively low temperatures (under 200-400° C., according to the chemical composition of the material), but it is released at higher temperatures. The equilibrium features of these materials in sorbing hydrogen are generally represented graphically by means of curves giving, at different temperatures, the equilibrium pressure of hydrogen over the NEG material as a function of the hydrogen concentration in the same material.
Advantageous features for a NEG material are a low activation temperature and, when hydrogen sorption is considered, a low hydrogen equilibrium pressure in the entire range of temperatures at which the material is to be used.
NEG materials especially suitable for hydrogen sorption are pure yttrium and an alloy disclosed in U.S. Pat. No. 3,203,901 which contains, by weight, 84% zirconium and 16% aluminum. Both of these materials require, however, relatively high temperatures for their activation, in the range of about 700-900° C. (depending on the degree of activation desired). British Pat. No. 1,248,184 and International Patent Application Publication No. WO 03/029502 disclose yttrium-rich materials, whose properties are essentially the same as those of pure yttrium. Another problem with the materials of British Pat. No. 1,248,184 is that the materials are essentially mixtures of pure metals, so that at high temperatures they can give rise to evaporation of the metal admixed with yttrium.
Another material widely employed for hydrogen sorption is an alloy having approximate composition, by weight, of 80% zirconium, 15% cobalt and 5% mischmetal (a commercial mixture of lanthanum and/or cerium and Rare Earths), disclosed in U.S. Pat. No. 5,961,750. This material has the drawback of a relatively high hydrogen equilibrium pressure at temperatures in excess of about 500° C.
Finally, International Patent Application Publication No. WO 2006/057020 discloses alloys containing zirconium (as the main component), yttrium and one or more elements chosen among aluminum, iron, chromium, manganese and vanadium for use in hydrogen sorption. The materials of International Patent Application Publication No. WO 2006/057020 have lower activation temperatures compared to the previously mentioned ones. However, they have very poor sorption characteristics for other gases, such as nitrogen.